2 week old Kittens

Hi,
My fosters are now 2 weeks and 4 days old. I have a question or two about them. The smaller of the two does eat as much as the larger one and more like she chews on the bottle. I try several time but she's not really interested like the other one. I also have concerns that she might have something wrong with her brain. When I hold them after their feedings she shakes like she's cold but I have her in a blanket on my chest with her sister(who doesn't do it). She does this a lot like she can't control it, if I pet her head or touch it she'll stop. Is this something I should take her to the vet for? She also is less mobile than her sister, hardly cries, and falls asleep faster. Is this just her personality? Anything helps these are my first bottle babies.
Thanks
Kat
and the Fosters...
Tyra...momma
Tallia and Tiara... babies

I also forgot to ask.. Tyra their momma is doing much better and her milk is pretty dried up(if you don't remember I had to take the babies away at 4 days after Tyra had to be put on meds for URI and an infection in her mammary glands-all of them). She is done with meds all clear and the masses are gone. When can I spay her? Thanks again.